function [xi,yi,ui,vi]=mirror(xo,yo,uo,vo);

% function mirror computes the image of the
% velocity data points for OA streamline anlysis

% computing image position
m1=-(-45.5+53)/7.5;
b1= 7.5 -m1*(-53);

m2=-1/m1; 
b2= yo - m2*xo;

r= sqrt( m1*m1 + 1);
d= (-m1*xo + yo - b1)/r;
b3= -d*r +b1;

xi=-(b2-b3)/(m2-m1);
yi= b3 + m1*xi;

%xi=xi-53;

% computing image velocity
th = atan(m2);
thv = atan(vo/uo);
vm=sqrt(uo*uo +vo*vo);

% 1st quadrant
if uo >=0 & vo >= 0,
 alf = th-thv;
 gam = pi + th + alf;
 ui=vm*cos(gam);
 vi=vm*sin(gam);

% 2nd quadrant
elseif uo < 0 & vo > 0,
 thv = thv +pi;
 alf = abs(th - thv);
 gam = pi +th -alf;
 ui=vm*cos(gam);
 vi=vm*sin(gam); 

% 3rd quadrant
elseif uo <=0 & vo < 0,
 alf=th-thv;
 gam =2*pi +th+alf;
 ui=vm*cos(gam);
 vi=vm*sin(gam);

% 4th quadrant
else
 th= th + pi;
 alf=abs(thv-th);
 gam=pi+th+alf;
 ui=vm*cos(gam);
 vi=vm*sin(gam);
end